1. Field of the Invention
The present invention relates to a torque distribution control device for a four-wheel drive vehicle, i.e., a device for controlling distribution of engine torque or power between the front and rear wheels of a four-wheel drive vehicle on the basis of a vehicle running condition.
2. Description of the Prior Art
An example of a prior art torque distribution control device for a four-wheel drive vehicle is disclosed in Japanese Patent Provisional Publication No. 61-169326 or 62-50231.
Such a prior art control device includes a multi-disc clutch incorporated in a front wheel drive line or rear wheel drive line for producing a desired transfer torque through control of a hydraulic pressure supplied thereto. The clutch engaging force, which is determined by the hydraulic pressure, increases as a detected rotational speed differential between the front and rear wheels increases. The ratio of the clutch engaging force (i.e., transfer torque) relative to the rotational speed differential between the front and rear wheels increases as the lateral acceleration of the vehicle increases.
A problem of the prior art torque distribution control device is that a control performed thereby is complicated. That is, with the prior art control device, it is necessary to obtain the rotational speed differential between the front and rear wheels by computation on the basis of the signals from vehicle wheel speed sensor, and vary the clutch engaging hydraulic pressure in response to a variation of the rotational speed differential so that a desired torque distribution is obtained. Accordingly, the control device is required to have a rapid responsiveness without causing any hunting.
Another problem is that the device is expensive since it requires a microcomputer having a high responsiveness, in order to exercise a complicated control and reduce a computation time.
A further problem is that the device is heavy and bulky, i.e., it requires a large space due to the provisions of a hydraulic pump, accumulator, etc.